FT4 Twin Pac

Fuel Control Upgrade

Plant Description

This is a remote start peaking power plant running a Pratt and Whitney FT4 Twin Pac on natural gas. Two engines are coupled to a common generator. The FT4s are run with water injection as well as exhaust into a SCR (selective catalytic reduction) system for NOX control. Sequencing is done with a Modicon Quantum, Fuel control is handled by 2 Hamilton Standard Stationary Power Controls (SPC), and Citect Scada is used as a HMI. Total generation capacity is 50 MW.

Problem Description

This Pratt and Whitney FT4 Twin Pac has long experienced difficulty in starting up and maintaining load. The old SPC fuel controllers had difficulty maintaining N3 speed stable enough for an auto synchronizer to close the generator breaker. Occasionally this would cause the plant to not make their required 10-minute startup timeframe. The non repeatability of the controls would also cause "mystery trips" shutting down one or both engines sometimes without a first out indication. Once synchronized, the plant would experience oscillations of N1 speed, MW load, TT7 temperature, and several other parameters across a 30 second time span. Poor loadshare control between the engines magnifies this instability.

Graph of FT4 combustion temperature

Graph of FT4 power production

The old water injection schematic has caused over injection of water resulting in destabilization and sometimes loss of flame. This has lead to dozens of full load trips within a period of weeks. Compounding this issue is a slow flame out detection schematic. This has proven particularly significant since the plant experienced a flame out event where the re-ignition of unburned fuel caused damage to the attached SCR system.

Work Performed

To solve the problems experienced by the plant, CSE Engineering removed the old SPC fuel controls and replaced each with a Woodward Atlas II. Woodward's GTC 250A GAP software was used with custom additions for loadshare between the FT4s, water injection, and TT7 thermocouple management. Along the fuel line a new Woodward Gas Shutoff Valve (GSOV) and Fuel Metering Valve (GS16) were added. The system was integrated to the existing Modicon Quantum and a Citect HMI project was developed for the new fuel control and functionality.

Results

Below is a Citect HMI trend of several parameters during a startup sequence. Oscillations of MW load, TT7 temperature, N1 speed and others have been eliminated. Generator frequency is quickly brought and held to within 2 RPM of setpoint making synchronization fast and reliable. The plant can now go from remote start to full power in less than 7 minutes.

Startup Graph After Upgrade (5min Timespan)

Additional improvements include: